Persistent Antenna Tuner Failure

The 2-by-20m double-zepp antenna with an AT-402 antenna tuner unit has been working flawlessly @DL2LSM for quite a long time. However, an automated AT-502 controller unit has been used to control the tuner during last years CQ WWDX CW 2013. The unit showed intermittent strange behavior since then. Eventually, the problem has been cured.

The Symptoms

The symptoms showed only when operating on 160m. There have been strong fluctuations in the reflected power as if there were arcing present. Careful analysis ruled out all components but the antenna tuner unit. The box has been opened but no direct cause has been visible. So, we suspected one or more of the capacitors of the matching network to be faulty. However, they all proved ok when testing with 220V AC. As a last measure the entire AT-402 tuner box has been exchanged for an AT-502 one.

AT-502 Defect: Destroyed Capacitor made of RG-58The problem disappeared for the moment. However, it returned when the unit has been switching its settings hot once with a power level of about 50W. This time the faulty component could easily been identified. See the adjacent picture. One of the capacitors made of RG-58 coaxial cable had been completely destroyed by arcing. A closer look revealed that the manufacturing quality of these capacitors was not as good as expected. Parts of the braid had been damaged and therefore not completely connected to the PCB trace.

The Cure

AT-502 Repaired: All RG-58 capacitors are replaced by RG-213 onesAll capacitors made of RG-58 have been replaced in both antenna tuner boxes by capacitors made of RG-213. Special care had been taken to ensure that the braid is fully connected. A ring made of solid wire is soldered around the entire braid to ensure this. RG-213 has almost the same capacity than RG-58. Therefore, the length of the new capacitors are in the same range and just fit into the tuner box.


Both antenna tuner boxes, an AT-402 and an AT-502, respectively,  have the capacitors exchanged. The fluctuations all disappeared and they both work very well again. A newly developed control box ensures that there won’t be any hot switching anymore.

Unfortunately, the process of isolating this fault has been very time consuming due to the fact that the antenna needed to be lowered and brought up quite often. Additionally, exchanging the tuner boxes required quite some cabling work since the interfaces are not compatible.


This entry was posted on Tuesday, May 20th, 2014 at 12:00 am and is filed under Technical Article. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

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